/**
 * @file
 * Packet buffer management
 *
 * Packets are built from the pbuf data structure. It supports dynamic
 * memory allocation for packet contents or can reference externally
 * managed packet contents both in RAM and ROM. Quick allocation for
 * incoming packets is provided through pools with fixed sized pbufs.
 *
 * A packet may span over multiple pbufs, chained as a singly linked
 * list. This is called a "pbuf chain".
 *
 * Multiple packets may be queued, also using this singly linked list.
 * This is called a "packet queue".
 * 
 * So, a packet queue consists of one or more pbuf chains, each of
 * which consist of one or more pbufs. CURRENTLY, PACKET QUEUES ARE
 * NOT SUPPORTED!!! Use helper structs to queue multiple packets.
 * 
 * The differences between a pbuf chain and a packet queue are very
 * precise but subtle. 
 *
 * The last pbuf of a packet has a ->tot_len field that equals the
 * ->len field. It can be found by traversing the list. If the last
 * pbuf of a packet has a ->next field other than NULL, more packets
 * are on the queue.
 *
 * Therefore, looping through a pbuf of a single packet, has an
 * loop end condition (tot_len == p->len), NOT (next == NULL).
 */

/*
 * Copyright (c) 2001-2004 Swedish Institute of Computer Science.
 * All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without modification,
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 *
 * Author: Adam Dunkels <adam@sics.se>
 *
 */

#include "lwip/opt.h"

#include "lwip/stats.h"
#include "lwip/def.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/pbuf.h"
#include "lwip/sys.h"
#include "arch/perf.h"

#include <string.h>

#define SIZEOF_STRUCT_PBUF        LWIP_MEM_ALIGN_SIZE(sizeof(struct pbuf))
/* Since the pool is created in memp, PBUF_POOL_BUFSIZE will be automatically
   aligned there. Therefore, PBUF_POOL_BUFSIZE_ALIGNED can be used here. */
#define PBUF_POOL_BUFSIZE_ALIGNED LWIP_MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE)

/**
 * Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type).
 *
 * The actual memory allocated for the pbuf is determined by the
 * layer at which the pbuf is allocated and the requested size
 * (from the size parameter).
 *
 * @param layer flag to define header size
 * @param length size of the pbuf's payload
 * @param type this parameter decides how and where the pbuf
 * should be allocated as follows:
 *
 * - PBUF_RAM: buffer memory for pbuf is allocated as one large
 *             chunk. This includes protocol headers as well.
 * - PBUF_ROM: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. Additional headers must be prepended
 *             by allocating another pbuf and chain in to the front of
 *             the ROM pbuf. It is assumed that the memory used is really
 *             similar to ROM in that it is immutable and will not be
 *             changed. Memory which is dynamic should generally not
 *             be attached to PBUF_ROM pbufs. Use PBUF_REF instead.
 * - PBUF_REF: no buffer memory is allocated for the pbuf, even for
 *             protocol headers. It is assumed that the pbuf is only
 *             being used in a single thread. If the pbuf gets queued,
 *             then pbuf_take should be called to copy the buffer.
 * - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from
 *              the pbuf pool that is allocated during pbuf_init().
 *
 * @return the allocated pbuf. If multiple pbufs where allocated, this
 * is the first pbuf of a pbuf chain.
 */
	struct pbuf *
pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
{
	struct pbuf *p, *q, *r;
	u16_t offset;
	s32_t rem_len; /* remaining length */
	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_alloc(length=%"U16_F")\n", length));

	/* determine header offset */
	offset = 0;
	switch (layer) {
		case PBUF_TRANSPORT:
			/* add room for transport (often TCP) layer header */
			offset += PBUF_TRANSPORT_HLEN;
			/* FALLTHROUGH */
		case PBUF_IP:
			/* add room for IP layer header */
			offset += PBUF_IP_HLEN;
			/* FALLTHROUGH */
		case PBUF_LINK:
			/* add room for link layer header */
			offset += PBUF_LINK_HLEN;
			break;
		case PBUF_RAW:
			break;
		default:
			LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
			return NULL;
	}

	switch (type) {
		case PBUF_POOL:
			/* allocate head of pbuf chain into p */
			p = memp_malloc(MEMP_PBUF_POOL);
			LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_alloc: allocated pbuf %p\n", (void *)p));
			if (p == NULL) {
				return NULL;
			}
			p->type = type;
			p->next = NULL;

			/* make the payload pointer point 'offset' bytes into pbuf data memory */
			p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + (SIZEOF_STRUCT_PBUF + offset)));
			LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
					((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
			/* the total length of the pbuf chain is the requested size */
			p->tot_len = length;
			/* set the length of the first pbuf in the chain */
			p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset));
			LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
					((u8_t*)p->payload + p->len <=
					 (u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
			/* set reference count (needed here in case we fail) */
			p->ref = 1;

			/* now allocate the tail of the pbuf chain */

			/* remember first pbuf for linkage in next iteration */
			r = p;
			/* remaining length to be allocated */
			rem_len = length - p->len;
			/* any remaining pbufs to be allocated? */
			while (rem_len > 0) {
				q = memp_malloc(MEMP_PBUF_POOL);
				if (q == NULL) {
					/* free chain so far allocated */
					pbuf_free(p);
					/* bail out unsuccesfully */
					return NULL;
				}
				q->type = type;
				q->flags = 0;
				q->next = NULL;
				/* make previous pbuf point to this pbuf */
				r->next = q;
				/* set total length of this pbuf and next in chain */
				LWIP_ASSERT("rem_len < max_u16_t", rem_len < 0xffff);
				q->tot_len = (u16_t)rem_len;
				/* this pbuf length is pool size, unless smaller sized tail */
				q->len = LWIP_MIN((u16_t)rem_len, PBUF_POOL_BUFSIZE_ALIGNED);
				q->payload = (void *)((u8_t *)q + SIZEOF_STRUCT_PBUF);
				LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
						((mem_ptr_t)q->payload % MEM_ALIGNMENT) == 0);
				LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
						((u8_t*)p->payload + p->len <=
						 (u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
				q->ref = 1;
				/* calculate remaining length to be allocated */
				rem_len -= q->len;
				/* remember this pbuf for linkage in next iteration */
				r = q;
			}
			/* end of chain */
			/*r->next = NULL;*/

			break;
		case PBUF_RAM:
			/* If pbuf is to be allocated in RAM, allocate memory for it. */
			p = (struct pbuf*)mem_malloc(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + offset) + LWIP_MEM_ALIGN_SIZE(length));
			if (p == NULL) {
				return NULL;
			}
			/* Set up internal structure of the pbuf. */
			p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF + offset));
            TRACE_BT("pbuf.payload %x %x %d\n", p, p->payload, SIZEOF_STRUCT_PBUF);
			p->len = p->tot_len = length;
			p->next = NULL;
			p->type = type;

			LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
					((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
			break;
			/* pbuf references existing (non-volatile static constant) ROM payload? */
		case PBUF_ROM:
			/* pbuf references existing (externally allocated) RAM payload? */
		case PBUF_REF:
			/* only allocate memory for the pbuf structure */
			p = memp_malloc(MEMP_PBUF);
			if (p == NULL) {
				LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 2, ("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_%s.\n",
							(type == PBUF_ROM) ? "ROM" : "REF"));
				return NULL;
			}
			/* caller must set this field properly, afterwards */
			p->payload = NULL;
			p->len = p->tot_len = length;
			p->next = NULL;
			p->type = type;
			break;
		default:
			LWIP_ASSERT("pbuf_alloc: erroneous type", 0);
			return NULL;
	}
	/* set reference count */
	p->ref = 1;
	/* set flags */
	p->flags = 0;
	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_alloc(length=%"U16_F") == %p\n", length, (void *)p));
	return p;
}


/**
 * Shrink a pbuf chain to a desired length.
 *
 * @param p pbuf to shrink.
 * @param new_len desired new length of pbuf chain
 *
 * Depending on the desired length, the first few pbufs in a chain might
 * be skipped and left unchanged. The new last pbuf in the chain will be
 * resized, and any remaining pbufs will be freed.
 *
 * @note If the pbuf is ROM/REF, only the ->tot_len and ->len fields are adjusted.
 * @note May not be called on a packet queue.
 *
 * @note Despite its name, pbuf_realloc cannot grow the size of a pbuf (chain).
 */
	void
pbuf_realloc(struct pbuf *p, u16_t new_len)
{
	struct pbuf *q;
	u16_t rem_len; /* remaining length */
	s32_t grow;

	LWIP_ASSERT("pbuf_realloc: sane p->type", p->type == PBUF_POOL ||
			p->type == PBUF_ROM ||
			p->type == PBUF_RAM ||
			p->type == PBUF_REF);

	/* desired length larger than current length? */
	if (new_len >= p->tot_len) {
		/* enlarging not yet supported */
		return;
	}

	/* the pbuf chain grows by (new_len - p->tot_len) bytes
	 * (which may be negative in case of shrinking) */
	grow = new_len - p->tot_len;

	/* first, step over any pbufs that should remain in the chain */
	rem_len = new_len;
	q = p;
	/* should this pbuf be kept? */
	while (rem_len > q->len) {
		/* decrease remaining length by pbuf length */
		rem_len -= q->len;
		/* decrease total length indicator */
		LWIP_ASSERT("grow < max_u16_t", grow < 0xffff);
		q->tot_len += (u16_t)grow;
		/* proceed to next pbuf in chain */
		q = q->next;
	}
	/* we have now reached the new last pbuf (in q) */
	/* rem_len == desired length for pbuf q */

	/* shrink allocated memory for PBUF_RAM */
	/* (other types merely adjust their length fields */
	if ((q->type == PBUF_RAM) && (rem_len != q->len)) {
		/* reallocate and adjust the length of the pbuf that will be split */
		q = mem_realloc(q, (u8_t *)q->payload - (u8_t *)q + rem_len);
		LWIP_ASSERT("mem_realloc give q == NULL", q != NULL);
	}
	/* adjust length fields for new last pbuf */
	q->len = rem_len;
	q->tot_len = q->len;

	/* any remaining pbufs in chain? */
	if (q->next != NULL) {
		/* free remaining pbufs in chain */
		pbuf_free(q->next);
	}
	/* q is last packet in chain */
	q->next = NULL;

}

/**
 * Adjusts the payload pointer to hide or reveal headers in the payload.
 *
 * Adjusts the ->payload pointer so that space for a header
 * (dis)appears in the pbuf payload.
 *
 * The ->payload, ->tot_len and ->len fields are adjusted.
 *
 * @param p pbuf to change the header size.
 * @param header_size_increment Number of bytes to increment header size which
 * increases the size of the pbuf. New space is on the front.
 * (Using a negative value decreases the header size.)
 * If hdr_size_inc is 0, this function does nothing and returns succesful.
 *
 * PBUF_ROM and PBUF_REF type buffers cannot have their sizes increased, so
 * the call will fail. A check is made that the increase in header size does
 * not move the payload pointer in front of the start of the buffer.
 * @return non-zero on failure, zero on success.
 *
 */
	u8_t
pbuf_header(struct pbuf *p, s16_t header_size_increment)
{
	u16_t type;
	void *payload;
	u16_t increment_magnitude;

	LWIP_ASSERT("p != NULL", p != NULL);
	if ((header_size_increment == 0) || (p == NULL))
		return 0;

	if (header_size_increment < 0){
		increment_magnitude = -header_size_increment;
		/* Check that we aren't going to move off the end of the pbuf */
		LWIP_ERROR("increment_magnitude <= p->len", (increment_magnitude <= p->len), return 1;);
	} else {
		increment_magnitude = header_size_increment;
#if 0
		/* Can't assert these as some callers speculatively call
		   pbuf_header() to see if it's OK.  Will return 1 below instead. */
		/* Check that we've got the correct type of pbuf to work with */
		LWIP_ASSERT("p->type == PBUF_RAM || p->type == PBUF_POOL", 
				p->type == PBUF_RAM || p->type == PBUF_POOL);
		/* Check that we aren't going to move off the beginning of the pbuf */
		LWIP_ASSERT("p->payload - increment_magnitude >= p + SIZEOF_STRUCT_PBUF",
				(u8_t *)p->payload - increment_magnitude >= (u8_t *)p + SIZEOF_STRUCT_PBUF);
#endif
	}

	type = p->type;
	/* remember current payload pointer */
	payload = p->payload;

	/* pbuf types containing payloads? */
	if (type == PBUF_RAM || type == PBUF_POOL) {
		/* set new payload pointer */
		p->payload = (u8_t *)p->payload - header_size_increment;
		/* boundary check fails? */
		if ((u8_t *)p->payload < (u8_t *)p + SIZEOF_STRUCT_PBUF) {
			LWIP_DEBUGF( PBUF_DEBUG | 2, ("pbuf_header: failed as %p < %p (not enough space for new header size)\n",
						(void *)p->payload,
						(void *)(p + 1)));\
				/* restore old payload pointer */
				p->payload = payload;
			/* bail out unsuccesfully */
			return 1;
		}
		/* pbuf types refering to external payloads? */
	} else if (type == PBUF_REF || type == PBUF_ROM) {
		/* hide a header in the payload? */
		if ((header_size_increment < 0) && (increment_magnitude <= p->len)) {
			/* increase payload pointer */
			p->payload = (u8_t *)p->payload - header_size_increment;
		} else {
			/* cannot expand payload to front (yet!)
			 * bail out unsuccesfully */
			return 1;
		}
	}
	else {
		/* Unknown type */
		LWIP_ASSERT("bad pbuf type", 0);
		return 1;
	}
	/* modify pbuf length fields */
	p->len += header_size_increment;
	p->tot_len += header_size_increment;

	LWIP_DEBUGF(PBUF_DEBUG, ("pbuf_header: old %p new %p (%"S16_F")\n",
				(void *)payload, (void *)p->payload, header_size_increment));

	return 0;
}

/**
 * Dereference a pbuf chain or queue and deallocate any no-longer-used
 * pbufs at the head of this chain or queue.
 *
 * Decrements the pbuf reference count. If it reaches zero, the pbuf is
 * deallocated.
 *
 * For a pbuf chain, this is repeated for each pbuf in the chain,
 * up to the first pbuf which has a non-zero reference count after
 * decrementing. So, when all reference counts are one, the whole
 * chain is free'd.
 *
 * @param p The pbuf (chain) to be dereferenced.
 *
 * @return the number of pbufs that were de-allocated
 * from the head of the chain.
 *
 * @note MUST NOT be called on a packet queue (Not verified to work yet).
 * @note the reference counter of a pbuf equals the number of pointers
 * that refer to the pbuf (or into the pbuf).
 *
 * @internal examples:
 *
 * Assuming existing chains a->b->c with the following reference
 * counts, calling pbuf_free(a) results in:
 * 
 * 1->2->3 becomes ...1->3
 * 3->3->3 becomes 2->3->3
 * 1->1->2 becomes ......1
 * 2->1->1 becomes 1->1->1
 * 1->1->1 becomes .......
 *
 */
	u8_t
pbuf_free(struct pbuf *p)
{
	u16_t type;
	struct pbuf *q;
	u8_t count;

	if (p == NULL) {
		LWIP_ASSERT("p != NULL", p != NULL);
		/* if assertions are disabled, proceed with debug output */
		LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 2, ("pbuf_free(p == NULL) was called.\n"));
		return 0;
	}
	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_free(%p)\n", (void *)p));

	PERF_START;

	LWIP_ASSERT("pbuf_free: sane type",
			p->type == PBUF_RAM || p->type == PBUF_ROM ||
			p->type == PBUF_REF || p->type == PBUF_POOL);

	count = 0;
	/* de-allocate all consecutive pbufs from the head of the chain that
	 * obtain a zero reference count after decrementing*/
	while (p != NULL) {
		u16_t ref;
		SYS_ARCH_DECL_PROTECT(old_level);
		/* Since decrementing ref cannot be guaranteed to be a single machine operation
		 * we must protect it. We put the new ref into a local variable to prevent
		 * further protection. */
		SYS_ARCH_PROTECT(old_level);
		/* all pbufs in a chain are referenced at least once */
		LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
		/* decrease reference count (number of pointers to pbuf) */
		ref = --(p->ref);
		SYS_ARCH_UNPROTECT(old_level);
		/* this pbuf is no longer referenced to? */
		if (ref == 0) {
			/* remember next pbuf in chain for next iteration */
			q = p->next;
			LWIP_DEBUGF( PBUF_DEBUG | 2, ("pbuf_free: deallocating %p\n", (void *)p));
			type = p->type;
			/* is this a pbuf from the pool? */
			if (type == PBUF_POOL) {
				memp_free(MEMP_PBUF_POOL, p);
				/* is this a ROM or RAM referencing pbuf? */
			} else if (type == PBUF_ROM || type == PBUF_REF) {
				memp_free(MEMP_PBUF, p);
				/* type == PBUF_RAM */
			} else {
				mem_free(p);
			}
			count++;
			/* proceed to next pbuf */
			p = q;
			/* p->ref > 0, this pbuf is still referenced to */
			/* (and so the remaining pbufs in chain as well) */
		} else {
			LWIP_DEBUGF( PBUF_DEBUG | 2, ("pbuf_free: %p has ref %"U16_F", ending here.\n", (void *)p, ref));
			/* stop walking through the chain */
			p = NULL;
		}
	}
	PERF_STOP("pbuf_free");
	/* return number of de-allocated pbufs */
	return count;
}

/**
 * Count number of pbufs in a chain
 *
 * @param p first pbuf of chain
 * @return the number of pbufs in a chain
 */

	u8_t
pbuf_clen(struct pbuf *p)
{
	u8_t len;

	len = 0;
	while (p != NULL) {
		++len;
		p = p->next;
	}
	return len;
}

/**
 * Increment the reference count of the pbuf.
 *
 * @param p pbuf to increase reference counter of
 *
 */
	void
pbuf_ref(struct pbuf *p)
{
	SYS_ARCH_DECL_PROTECT(old_level);
	/* pbuf given? */
	if (p != NULL) {
		SYS_ARCH_PROTECT(old_level);
		++(p->ref);
		SYS_ARCH_UNPROTECT(old_level);
	}
}

/**
 * Concatenate two pbufs (each may be a pbuf chain) and take over
 * the caller's reference of the tail pbuf.
 * 
 * @note The caller MAY NOT reference the tail pbuf afterwards.
 * Use pbuf_chain() for that purpose.
 * 
 * @see pbuf_chain()
 */

	void
pbuf_cat(struct pbuf *h, struct pbuf *t)
{
	struct pbuf *p;

	LWIP_ERROR("(h != NULL) && (t != NULL) (programmer violates API)",
			((h != NULL) && (t != NULL)), goto errout;);

	/* proceed to last pbuf of chain */
	for (p = h; p->next != NULL; p = p->next) {
		/* add total length of second chain to all totals of first chain */
		p->tot_len += t->tot_len;
	}
	/* { p is last pbuf of first h chain, p->next == NULL } */
	LWIP_ASSERT("p->tot_len == p->len (of last pbuf in chain)", p->tot_len == p->len);
	LWIP_ASSERT("p->next == NULL", p->next == NULL);
	/* add total length of second chain to last pbuf total of first chain */
	p->tot_len += t->tot_len;
	/* chain last pbuf of head (p) with first of tail (t) */
	p->next = t;
	/* p->next now references t, but the caller will drop its reference to t,
	 * so netto there is no change to the reference count of t.
	 */
	return;

	errout:
	printf("\n");
	return;
}

/**
 * Chain two pbufs (or pbuf chains) together.
 * 
 * The caller MUST call pbuf_free(t) once it has stopped
 * using it. Use pbuf_cat() instead if you no longer use t.
 * 
 * @param h head pbuf (chain)
 * @param t tail pbuf (chain)
 * @note The pbufs MUST belong to the same packet.
 * @note MAY NOT be called on a packet queue.
 *
 * The ->tot_len fields of all pbufs of the head chain are adjusted.
 * The ->next field of the last pbuf of the head chain is adjusted.
 * The ->ref field of the first pbuf of the tail chain is adjusted.
 *
 */
	void
pbuf_chain(struct pbuf *h, struct pbuf *t)
{
	pbuf_cat(h, t);
	/* t is now referenced by h */
	pbuf_ref(t);
	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_FRESH | 2, ("pbuf_chain: %p references %p\n", (void *)h, (void *)t));
}

/**
 * Dechains the first pbuf from its succeeding pbufs in the chain.
 *
 * Makes p->tot_len field equal to p->len.
 * @param p pbuf to dechain
 * @return remainder of the pbuf chain, or NULL if it was de-allocated.
 * @note May not be called on a packet queue.
 */
	struct pbuf *
pbuf_dechain(struct pbuf *p)
{
	struct pbuf *q;
	u8_t tail_gone = 1;
	/* tail */
	q = p->next;
	/* pbuf has successor in chain? */
	if (q != NULL) {
		/* assert tot_len invariant: (p->tot_len == p->len + (p->next? p->next->tot_len: 0) */
		LWIP_ASSERT("p->tot_len == p->len + q->tot_len", q->tot_len == p->tot_len - p->len);
		/* enforce invariant if assertion is disabled */
		q->tot_len = p->tot_len - p->len;
		/* decouple pbuf from remainder */
		p->next = NULL;
		/* total length of pbuf p is its own length only */
		p->tot_len = p->len;
		/* q is no longer referenced by p, free it */
		LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_STATE, ("pbuf_dechain: unreferencing %p\n", (void *)q));
		tail_gone = pbuf_free(q);
		if (tail_gone > 0) {
			LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_STATE,
					("pbuf_dechain: deallocated %p (as it is no longer referenced)\n", (void *)q));
		}
		/* return remaining tail or NULL if deallocated */
	}
	/* assert tot_len invariant: (p->tot_len == p->len + (p->next? p->next->tot_len: 0) */
	LWIP_ASSERT("p->tot_len == p->len", p->tot_len == p->len);
	return ((tail_gone > 0) ? NULL : q);
}

/**
 *
 * Create PBUF_RAM copies of pbufs.
 *
 * Used to queue packets on behalf of the lwIP stack, such as
 * ARP based queueing.
 *
 * @note You MUST explicitly use p = pbuf_take(p);
 *
 * @note Only one packet is copied, no packet queue!
 *
 * @param p_to pbuf source of the copy
 * @param p_from pbuf destination of the copy
 *
 * @return ERR_OK if pbuf was copied
 *         ERR_ARG if one of the pbufs is NULL or p_to is not big
 *                 enough to hold p_from
 */
	err_t
pbuf_copy(struct pbuf *p_to, struct pbuf *p_from)
{
	u16_t offset_to=0, offset_from=0, len;

	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 3, ("pbuf_copy(%p, %p)\n",
				(void*)p_to, (void*)p_from));

	/* is the target big enough to hold the source? */
	LWIP_ERROR("pbuf_copy: target not big enough to hold source", ((p_to != NULL) &&
				(p_from != NULL) && (p_to->tot_len >= p_from->tot_len)), return ERR_ARG;);

	/* iterate through pbuf chain */
	do
	{
		LWIP_ASSERT("p_to != NULL", p_to != NULL);
		/* copy one part of the original chain */
		if ((p_to->len - offset_to) >= (p_from->len - offset_from)) {
			/* complete current p_from fits into current p_to */
			len = p_from->len - offset_from;
		} else {
			/* current p_from does not fit into current p_to */
			len = p_to->len - offset_to;
		}
		MEMCPY((u8_t*)p_to->payload + offset_to, (u8_t*)p_from->payload + offset_from, len);
		offset_to += len;
		offset_from += len;
		LWIP_ASSERT("offset_to <= p_to->len", offset_to <= p_to->len);
		if (offset_to == p_to->len) {
			/* on to next p_to (if any) */
			offset_to = 0;
			p_to = p_to->next;
		}
		LWIP_ASSERT("offset_from <= p_from->len", offset_from <= p_from->len);
		if (offset_from >= p_from->len) {
			/* on to next p_from (if any) */
			offset_from = 0;
			p_from = p_from->next;
		}

		if((p_from != NULL) && (p_from->len == p_from->tot_len)) {
			/* don't copy more than one packet! */
			LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
					(p_from->next == NULL), return ERR_VAL;);
		}
		if((p_to != NULL) && (p_to->len == p_to->tot_len)) {
			/* don't copy more than one packet! */
			LWIP_ERROR("pbuf_copy() does not allow packet queues!\n",
					(p_to->next == NULL), return ERR_VAL;);
		}
	} while (p_from);
	LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE | 1, ("pbuf_copy: end of chain reached.\n"));
	return ERR_OK;
}

/**
 * Copy (part of) the contents of a packet buffer
 * to an application supplied buffer.
 *
 * @param buf the pbuf from which to copy data
 * @param dataptr the application supplied buffer
 * @param len length of data to copy (dataptr must be big enough)
 * @param offset offset into the packet buffer from where to begin copying len bytes
 */
	u16_t
pbuf_copy_partial(struct pbuf *buf, void *dataptr, u16_t len, u16_t offset)
{
	struct pbuf *p;
	u16_t left;
	u16_t buf_copy_len;
	u16_t copied_total = 0;

	LWIP_ERROR("netbuf_copy_partial: invalid buf", (buf != NULL), return 0;);
	LWIP_ERROR("netbuf_copy_partial: invalid dataptr", (dataptr != NULL), return 0;);

	left = 0;

	if((buf == NULL) || (dataptr == NULL)) {
		return 0;
	}

	/* Note some systems use byte copy if dataptr or one of the pbuf payload pointers are unaligned. */
	for(p = buf; len != 0 && p != NULL; p = p->next) {
		if ((offset != 0) && (offset >= p->len)) {
			/* don't copy from this buffer -> on to the next */
			offset -= p->len;
		} else {
			/* copy from this buffer. maybe only partially. */
			buf_copy_len = p->len - offset;
			if (buf_copy_len > len)
				buf_copy_len = len;
			/* copy the necessary parts of the buffer */
			MEMCPY(&((char*)dataptr)[left], &((char*)p->payload)[offset], buf_copy_len);
			copied_total += buf_copy_len;
			left += buf_copy_len;
			len -= buf_copy_len;
			offset = 0;
		}
	}
	return copied_total;
}
